Sextant



Patented Nov. `12, 1940 PATENT GFFI'CE SEXTANT samuel M. Burke. Dayton,ohio, and cari J.

Crane, Montgomery, Ala.

Application March z5, 1939, 4serial No. 264,1os

7 Claims.

(Granted under the act of March 3, 1883, as

amended April 30, 1928; 370 Q. G. 757) The invention described hereinmay be manufactured and used by or for the Government for governmentalpurposes, without the payment to us of any royalty thereon.

This invention relates to a sextant having a`n articial horizon orreference mark. gyroscopically stabilized so that it will not beaffected by vibration or by short period acceierations of the instrumentor the vehicle such as anairplane in lo which the instrument is used.

In the drawing,

Fig. l is a side view showing the sextant partly in cross-section andpartly in elevation;

Fig. 2 is an elevation looking at the sextant from the right in Fig. 1;

Fig. 3 is a cross-section along the line 3--3 of Fig. 1; and

Fig. 4 is a plan view'of the reticle.

Referring to Fig. 1, the casing 'I has rigidly n mounted therein supportbearings I which pivotally supporta gimbal ring II upon which in turnare rigidly mounted bearings I2 similar to bearings I0 for pivotallymounting ,housing I carrying reticle 2 by means of arm 24. Gyrorotor I4,preferably of the self erecting type, is carried with its spin axisvertical inside housing I by bearings I3.y The gyra-rotor is caused tospin about its vertical axis by conventional means, V not shown, sinceit does not constitute apart of 30 the invention. Y As is well known,compressed air mechanism 4may be used. Swinging movement of reticle 2 islimited by a projection 8 on housing I cooperating with cup 9 onhousing 1. Light 22 suitably wired to a source of electric power, not

33 shown, and to switch 23, illuminates cross-hairs or other referencemark on reticle 2, the'` outlines of which are reected by sllveredmirror 3 through lens 6 and index glass or plate 4 to eyepiece 5. Theimage of the celestial body or object n whose altitude it isv desired tomeasure enters the sextant through shading glass 25 and is reflected toeyepiece 5,by index glass 4. Coincidence of the images of\ the `observedbody and the crosshairs of the reticle is obtained by rotating index lglass 4 by means of knob I9, the. angle of rotation of the index glassbeing a measure of the altitude of the celestial body. Index plate 4 ispivotally mounted in casing l by shaft I and carries a pointer which maybe observed through a wine 50 dow in the casing for reading the degreesof angle of tilt of plate 4 on degree scale 2|. Plate 4 is rotated byknob I8, shaft I8 and gear I1 cooperating with gear I6 fixed to plate 4.Minutes of elevation of plate 4 may be read by a suitable -66 scaleonknob I8. The index plate 4 and angle 'instrument and of the airplane.

reading scales are zeroed so that the pointer 2 reads zero when theplate is tilted to4 a position where horizontal light rays, parallel tothe artiiicial horizon established by reticle 2, will be reflected tothe eyepiece. Thus it is seen that the 5 angle which the observed bodymakes with the artificial horizon is read directly on the scale,

permitting accurate determination of the altiof motion of the twoimages, the lens 6 must have 15 a focal length equal rto the distance ofthe reticle from the gimbal axis of the gyro and the reticle must be atthe principal focus of the lens. Since the focal length 'of lens 6 isequal .to the radius of tilt of reticlejZ, the image'of the reticle asob- 20 served in the eyepiece will maintain its position in spacerelative to an observed celestial body regardless of limited tilting oftheinstrument about ,I

a transverse horizontal axis. In order to read the altitude of anobserved body, it is only necesg5 sary to bring the celestial body andthe reference 'mark on the reticle into coincidence in the `iield ofview of the eyepiece 5 and within the operative pivotal range of reticle2. As shown in Fig. 1 of the drawing, lens 6 transmits an image of the 3reticle through the index plate and to the eyepiece by parallel bundlesof rays. @Due to the long period of the gyro. the artificial horizonformed by reticle 2' is, unlike a bubble or pendulum, unaiected byvibration or by the short period accelerationsproduced by movement ofthe The instrument is free from precsion due to the fact that thereticle is a part of the gyro housing itself and no Work or loads,frictional or o c otherwise are imposed on the gyro in securingstabilization of the reticle.

It is to be understood that incidental features such as light 22 willhavesuitable wiring, that necessary connections will be made forsupplying compressed air to the gyro if air driven or electricalconnections if electrically driven. Also a conventional lock or latchmay be provided to hold gyro I4 in locked inoperative position. Thesefeatures are old and well known and form 5g no part of the presentinvention.

' In using the device, the observer at rst brings the device intooperative position with the reticle approximately horizontal, operatesthe necessary switches or valves to start the .gyroto rotating.

Light from 22 will illuminate cross-hairs in reticle 2 of which theimage will be observable in eyepiece 5 by means of the optical systemconsisting of mirror 3 and lens 6. Index plate 4 is rotated from itszero position by means of knob I9 until an image of the observedcelestial body is reilected into coincidence with the image of thecross-hairs observed in the eyepiece.

- through which the plate is rotated is the altitude of the observedbody. It will be readily appreciated that the establishment ofcoincidence of the images of the cross-hairs and the body will give anaccurate determination of the altitude of the body regardless of thehorizontal tilting of the sextant. Lateral tilting of the sextant iscontrollable by the observer and, furthermore, has little effect uponthe determination.

Details of construction may be altered or omitted Without departing fromthe spirit of the invention and it is obvious that changes may be madethat fall within the scope of the appended claims. e

We claim:

l. A sextant comprising a casing, an eyepiece, a rotatably mounted indexplate adapted to reect and transmit images, a gimbal ring pivotallymounted in said casing, a rotor housing pivotally mounted in said gimbalring, a gyro-rotor of the self-erecting -type mounted in said rotorhousing to spin about a vertical axis for stabilizing said housing, areticle carried by said housing at a predetermined distance from thesaid pivotal mounting of said rotor housing to represent an artiicialhorizon, an optical system for reilecting the reference mark of saidarticial horizon to said eyepiece, the focal length1 of said opticalsystem being equal to said predetermined distance, means for rotatingsaid index plate to produce coincidence of the reference mark of saidarticial horizon with a body reflected by said index plate to saideyepiece.

2. A sextant comprising an eyepiece, a rotatably mounted index plateadapted to reflect and transmit images, a pivotally mounted reticlerepresenting an articial horizon and having reference means thereonobservable through said eyepiece, said reticle being spaced apredetermined distance from its pivotal mounting, a precessionfree,self-erecting gyrescope for stabilizing said reticle, an optical systemhaving a focal length equal to said predetermined distance fortransmitting arf image of said reference means to said eyepiece, andmeans for rotating said index lglass to establish coincidence of thereference mark of said articial horizon with a celestial body reflectedby said index glass to said eyepiece. 3. In a gyro sextant, a rotatablyvmounted index plate adapted to reflect and transmit images. agyroscopically stabilized, pivotally mounted reticle spaced apredetermined distance from said The angle I pivotal mounting, anoptical system for transmitting by parallel bundles of rays, an image ofsaid reticle through said index plate to said eyepiece, the focal lengthof said optical system being equal to the said predetermined distance,and means to rotate said index plate to cause said image to coincidewith an image of a celestial body reected from said index plate to saideyepiece.

4. A sextant comprising 'a casing having an eyepiece, a gimbal ringpivotally mounted in said casing about a substantially horizontal axis,a rotor housing mounted in said gimbal ring to pivot about asubstantially horizontal axis at right angles to'said rst horizontalaxis, a gyrorotor mounted in :aid rotor housing to rotate about avertical axis, a reticle carried by said rotor housing, a reference markon said reticle observable in said eyepiece, an index plate mounted insaid casing to rotate about a transverse horizontal axis, scale means toindicate the angle which the indexv plate makes with respect to thereticle when stabilized, said index plate being arranged to reiiect animage of an observed body into coincidence with the reference mark ofthe reticle when observed through said eyepiece.

5. In a gyro sextant, a rotatably mounted index plate adapted to r'eectand transmit images, a gyroscopically stabilized, pivotally mountedreticle arranged to be illuminated, -said reticle being spaced apredetermined distance from said pivotal mounting, an optical system fortransmitting animage of said reticle through said index plate to saideyepiece, the focal length of said optical system being the same lengthas the said predetermined distance of said reticle from said pivotalmounting, and means for rotating said index plate to cause said image tocoincide with an image of a celestial body reilected from said indexplate to said eyepiece.

6. In a gyro sextant, an optical system including a lens having apredetermined focal length, an eyepiece and an index mirror for sightinga heavenly body', a reticle disposed in said optical system, asubstantially precessionfree gyroscope of the self-erecting type forsupporting and stabilizing said reticle about ltwo axes, said reticlebeing spaced from one of said axes by an amount equal to said focallength, whereby the center of said reticle may be made to coincide withthe image of said heavenly body.

7'. A device as recited in claim 6 and further including reference meanson said reticle and scale means -associated with said index plate,coincidence of said reference means and said heavenly body as observedthrough said eyepiece being readable on said scale as the altitude ofthe heavenly body regardless of limited tilting of said

